This invention relates to the systems for inspection of substrates, especially semiconductor wafers and reticles. More specifically, the invention relates to a novel system which illuminates lines of pixels on the substrate, and images reflected and/or scattered light from the lines.
Several systems are known in the art for the inspection of wafers and reticles. Two examples of such systems are depicted in FIGS. 1 and 2. In the system exemplified in FIG. 1, the wafer 100 is illuminated with a light beam emanating from a light source 110 and reaching the wafer at 90xc2x0 angle (generally referred to as normal illumination). Preferably, light source 100 provides coherent light, i.e., source 100 may be a laser source. The light beam is scanned over the wafer by a scanner 120, typically an acousto-optical scanner (AOD) or a rotating mirror, in the direction marked by the double-headed arrow. The wafer 100 is moved in the perpendicular direction by moving the stage upon which the wafer rests. Thus, a two dimensional area of the wafer can be scanned by the light beam.
Since the wafer has basically a mirror-like top surface, the light beam specularly reflects back per Snell""s law at 180xc2x0. This specularly reflected light is collected by a light sensor 140 and its signal is used to obtain a xe2x80x9cbright fieldxe2x80x9d image, i.e., an image created from specularly reflected light. However, whenever the light beam hits an irregularity on the wafer, such as a particle or etched pattern, the light scatters in various directions. Some of the diffracted/scattered light is then collected by the light sensors 130, and their signal is used to obtain a xe2x80x9cdark fieldxe2x80x9d image, i.e., an image created from diffracted/scattered light. Thus, when the wafer has no pattern on it (e.g., blank wafer), the irregularities appear in the dark field image as stars in a dark sky. When the wafer has a pattern on it, the irregularities cause a scattered light which deviates from the normal diffraction caused by the pattern.
In the system exemplified in FIG. 2, the wafer 200 is illuminated by a light beam emanating from light source 210, but reaching the wafer at a shallow angle, generally referred to as grazing illumination. The light beam is scanned over the wafer by a scanner 220, typically an acousto-optical scanner or a rotating mirror, in the direction marked by the double-headed arrow. The wafer 200 is moved in the perpendicular direction by moving the stage upon which the wafer rests. Thus, a two dimensional area of the wafer can be scanned by the light beam.
Since the light reaches the wafer at a grazing angle xcex8, its specular reflection is at a corresponding angle, xcex8xe2x80x2, according to Snell""s law. This light can be collected by sensor 240, and its signal is used to create the bright field image. Any diffracted/scattered light is collected by sensors 230, the signal of which is used to create dark field images.
It should be appreciated that in the above exemplified systems, with respect to each sensor the image data is acquired serially. That is, each two dimensional image, whether bright or dark field, is constructed by acquiring signals of pixel after pixel, per the scanned light beam. This is time consuming serial operation, which directly affects the throughput of such systems. Moreover, the scan speed of such systems is limited by the scanner""s speed (i.e., the band-width for an acousto-optic scanner) and by the electronics that support the detectors, e.g., the PMT (Photo-Multiplier Tube). Thus, a need exists to develop a system that does not utilize a scanned light beam.
Looking forward, as design rules shrink, the importance of detecting increasingly small irregularities becomes paramount. With design rules such as 0.18 and 0.15 xcexcm, very small irregularities, such as particles of sub-micron size, can be killer defects and cause the device to malfunction. However, in order to detect such small irregularities, one needs to use a very small wavelength light source, such as ultra violate (UV) or deep ultra violate (DUV) light source. This presents at least two crucial problems: first, optical elements operating in the DUV regime are expensive and, second, small short wave implies small spot size of the light beam; thus, the scanning speed and collection data rate need to be increased.
Currently, commercially available AOD that can support a scanning for a DUV beam have a very limited performance. Additionally, even if such AOD can be developed, at present it is unclear whether it could withstand the energy levels required for obtaining a high resolution image using a DUV light beam. Thus, reducing the power level may also dictate use of a slower scanning AOD. Therefore, future systems may also require implementations that do not relay on beam scanning.
According to the present invention, the wafer is illuminated by an elongated xe2x80x9clinearxe2x80x9d or xe2x80x9cline spot.xe2x80x9d The xe2x80x9cline spotxe2x80x9d is basically an elongated illumination on the wafer surface, such that it covers several pixels aligned to form a line. In the preferred embodiment, the number of pixels in the linear spot is on the order of thousands. The linear spot is held stationary with respect to one direction, but the wafer is scanned under it in the other direction. Thus, a two dimensional area is covered and can be imaged. Imaging is preferably performed using a sensor array, such as a line CCD. In the preferred embodiment, two linear spots are used in conjunction with two line CCD""s. The detected lateral pixel size (along the narrow dimension of the line) is determined by the illumination line width. The detected pixel size along the longitudinal direction is determined by the resolution of the collection optics and the line CCD camera pixel size. When inspecting a patterned wafer, the linear spots are projected at complementing 45xc2x0 angles to the xe2x80x9cstreets and avenuesxe2x80x9d axis of the wafer.
The present invention is advantageous in that it enables much faster data acquisition rate. Furthermore, it is operable with short wavelengths, such as UV or deep UV illumination. Notably, the inventive system does not require a scanning mechanism. Other advantages of the invention will appear as the description proceeds.